Determination of irreducible water and other properties of core samples



May 25, 1954 5, MESSER 2,679,159

DETERMINATION OF IRREDUCIBLE WATER AND OTHER PROPERTIES OF CORE SAMPLESFiled Jan. 21, 1950 2 Sheets-Sheet 1 INTOR. ZLMEI? 5. #7555512.

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E. S. DETERMINATION OF IRREDUCIBLE WATER AND y 5, 1954 MESSER OTHERPROPERTIES OF CORE SAMPLES 2 Sheets-Sheet 2 Filed Jan. 21,. 1950figtvmtti 8 an Oh Id O M OM mN ON INVENTOR.

ELMZA 5. H5565 BY GTTC/l VEYS- Patented May 25, 1954 .DETERMINATION OFIRREDUCIBLE WATER AND OTHER PROPERTIES OF CORE SAM- PLES Elmer:S.;Messer, Ponoa City, Okla., 'assignor to Continental .Oil Company,Ponca :City, 0kla., a

corporation of Delaware Application January 21, 1950,'Serial No."139,839

.8Claims.

, 1 T'I'he present inventionrelates amethod for adetermining :the:irreducible interstitial water content, capillary pressure, and relatedchar- :acteristics of cores or :samples of .zsub-surface 'formations.

in sorder to obtain :a more'rcomplete analysis of .coresamples :todetermine the petroleum reserves, :it :is necessary and .essential todetermine the irreducible interstitial watercontent of the formation.This quantity is generally referred "to :as a ::percentage *of theavailable 51),01'8 volume "of the nore thatisloccupied by water.

The term interstitial water is defined .as that rwaterth'at iCOeXiStS inthe pore space with the nil :prior to exploitation. The irreducibleinter- 'StitidlfWflliBI' content of the formation is that "which islheldin *a state -of equilibrium with the nil by capillary pressure in all ofthat .zone of the forma'tiomwhich sufliciently tar above the water tableto be :abovethe transition zone. .The .water content ofthe transitionzone is also .a

asynonymously with these terms; .however, this ageclogicall-y, connatewater is that which was entrapped in the pore spaces of the rock at thetime -of :deposition. :Forethe sake :of brevity throughout thespecification and claims, ir-

-reducib1e interstitial water content will be referred to as theirreducible water.

:Since the irreducible water in the .-formation above the transition"zone is dependent -on the mock structure, .it is a :function of thecapillary pressure; and several techniques have 'been developed tomeasure this pressure and its degree of Water saturation.

'Themethods'used to determine the capillary ,pressure and theirreducible water may be divide'dinto three classifications. Themostprominent-of these :is generally referred .to as the restored statemethod. The techniqueis described --in detail in the literature andessentially consists of saturating a core sample with the reser-'voirsfluid'and placing it in contact'withaporous mem'brane saturatedwith the samefluid. Oil,

gas or another liquid under pressure :is then 'applied to the exposedarea :of the :core and :one

side of the membrane driving the saturant-irom The interstitial watercontent and 2 thelpores of thesample. The .membrane is such that it willpass the saturating fluid but is .impervious to the driving liquid orgas. At each applied pressure, an equilibrium saturation is obtained thecore is weigl'ied to determine the fluid content.This'pressure-saturation .rep-

*resents one point :of a curve generally known .as a capillary pressurecurve. With increased pressures there is adecrease .in saturation .untilthe irreducible water is attained "when there is no longer anappreciable change in the liquid content. The amount of liquid remainingin the core represents the irreducible liquid content and correspondsto-arelated capillary pressure.

This method has :certain limitations since the core sample must :have aflat 'face .for complete contact with the membrane; the .amount of ap-:plied pressure :is limited by -the capacity of the membrane.

Since at every pressure there is a time required to attain anequilibrium, ;a :period of several days is usually required "to obtainthe :data for thelcapill'ary pressure curve and the irreducible vvater;value.

The second classification includes the method of :determining thecapillary :pressure by :cenitrifuga'l'iorce. v'Ihismethool ofdetermination :is essentially similar to the restored .state .method,:exceptxthat :the :force driving'the liquid from the :pore spaces is :acentrifugal force. .In the determination, the speed of rotation of thesample is increased in steps until "a'speedrreached where any increasedoes not appreciably lower theawatercontent nf'the core. The Waterdriven rout-is measured-in atpiipette; :and in the final vdetermination,the loss of weight in the :core is compared with the water collected -inthis pi- .rpette. This method is much faster than the :restored statemethod; however, it is limited in that the speed of rotation must beconstant and any wariation will nullify the test. In'thistmeth- :Ud, thesample must have a'fiat face for :contact with the semi-perviousimernbrance. :Calculation of the capillary force :from the rotatingspeed is long and involved.

The third method ofmeasuring capillary pressure is based on the force"necessary to drive "mercury into "the pore space of the core "sample.This method utilizes the variation "in forces necessary to change thesaturation of aporous medium with mercury. The main disadvantages ofthis method are the core samples used 'arenot suitable for'furthertesting; mechanical failures and corrections for the apparatus arenumerous.

:All of the foregoing :procedures, as well as my own new method to bedescribed hereinafter, usually require that the core sample be firstcleaned to remove the crude oil and related organic substances which maybe present in the sample. A convenient way of cleaning, commonly used,is to extract the organic material with carbon tetrachloride in aSoxhlet extractor.

The principal object of my invention is to provide a method fordeterming the irreducible water with speed and accuracy.

Another object of my invention is to provide a method havingcalculations that are simple and direct.

Still another object is to provide a method which does not depend upon auniformly sized and shaped sample of material and that does not alterthe structure or characteristics of such sample for further testing.

Other objects and advantages will become apparent as the followingdescription proceeds.

To the accomplishment of the foregoing and related ends, said inventionthen comprises the features hereinafter fully described and particularlypointed out in the claims, the following description and the annexeddrawings setting forth in detail certain illustrative embodiments of theinvention, these being indicative, however, of but a few of the variousWays in which the principle of the invention may be employed.

Broadly stated then, this invention comprises a method for determininthe irreducible water of a core sample of the sub-surface stratacomprising the steps of saturating said sample with a vaporizableliquid, and while evaporating the liquid from said sample, periodicallynoting the decreasing weight of said sample during evaporation andsubsequently determining the irreducible water of said sample from agraphical interpretation of the evaporation time-weight changerelationship.

More specifically stated, this invention comprises a method fordetermining the irreducible water of a core sample taken from thesub-surface strata comprising the steps of saturating said core samplewith a liquid evaporating the liquid from said saturated sample,determining from the inflection of the rate of evaporation curve thepoint at which only irreducible water remains, and calculatingsubsequently the actual percentage of the pore space of said sampleoccupied by the irreducible water.

In the further explanation of my improved method, it becomes convenientto illustrate apparatus by which such method may be carried on; twoforms of such apparatus are illustrated in the drawings in which:

Fig. 1 is a diagrammatic illustration of one iform of apparatus that maybe used in my inven- Fig. 2 is a diagrammatic illustration of anotherform of apparatus that may be used in the practice of my invention; and

Fig. 3 illustrates a typical curve showing liquid in sample versus time.

Fig. 4 illustrates capillary pressure vs. interstitial water content.

Referring now more particularly to the drawings, Fig. 1 illustrates oneform of apparatus, a commercially available automatic balance, which maybe used for practicing the present invention. Core sample It is firstsaturated preferably with a liquid that does not leave a residue whenevaporated or cause any chemical action with the sample itself.

Examples of some of the liquids used and found satisfactory are Water,toluene, tetrachloroethane, benzene, and normal propyl alcohol. It ispossible also to use liquids which have a vapor pressure greater thanatmospheric at room.temperatures. Examples of such liquids are a widevariety of petroiemn fractions and their derivatives. Liquified propaneis an example of such a liquid which will be found satisfactory for use.It will be understood, of course, that the time necessary to reach theirreducible liquid content is a function of the evaporation rate of theparticular liquid used.

The reservoir fluid would, of course, be the most appropriate to use;but, since it contains salts in solution, compensation must be made forthe amount of salts deposited in the pores after evaporation. Thisrequires extra care and calculation. Distilled water, resembling mostnearly the properties of the reservoir fluid, may be used on certainrock samples; however, on some structures, principally those of shaly orclay nature,

' it causes a swelling of the sample and a change of itscharacteristics.

Any of the liquids named above can be used, however, there should be acorrection applied to the volume retained depending on the nature of theliquid, other than water used. This correction is a ratio of theabsorption factor of the crystals to the liquid used as compared to thatof water. When the core sample consists principally of CaCOx crystals,the correction factors for the various liquids are: 1.24 for toulene,1.24 for benzene, 1.16 for tetrachloroethane, and 1.75 for normal propylalcohol. If the principal core component is siliceous material then thecorrection factors are: 1.11 for toluene, 1.13 for benzene, and 1.11 fortetrachloroethane. A simple test to determine the principal constituentsof the sample is the placing of a drop of dilute hydrochlorid acid onthe rock. A vigorous evolution of carbon dioxide means an abundance ofCaCOs, while no reaction means the principal coonstitucut is that ofsiliceous material. Since most core samples are made up of a combinationof the two types, an average correction factor can be used for routinepurposes. A value of the irreducible water as determined by using thevarious fluids will be shown later.

Figure 1 shows one form of balance I which may be used in carrying outmy invention. It consists of a beam 2, a fulcrum 3, a pan 4, a weightpan 5, weights 8, and a mirror 1 onto which is projected a beam of lightfrom a light source 8 and which beam is then reflected onto a scale 9.

The balance of the type illustrated here is in equilibrium with its fullset of weights in place on platform 5 before it is put to use.

The liquid treated core sample It is placed in a tube H usually ofglass, and tube H is placed on platform 4 of balance i. A sufiicientnumber of suitable weights are then removed from platform a which willcorrespond to the weight of the prepared sample and tube placed upon pan4; the balance is thereby brought to equilibrium after which theperiodic measurement of evaporation weight loss is begun. A stream ofgas, ordinarily air from tube 12 is now passed over and around coresample it with only enough velocity to effect a rapid evaporation butnot enough to agitate the balance so that a reading might be hard toobtain. Gases other than air may also be used with favorable results.Examples of these gases are nitrogen, carbon dioxide, neon, and othersthat do not react with the sample.

.The stream of airpr gaS'lPQSSiIIgRDVGI the evaporated from the sample.This continuous v decrease inweight is noted-on the scale 9. There:willbe a markeddecrease in the rate of evaporation at a point where theforces "causing 'the 1 weight loss by evaporation begin to be overcome.by the -.capillary or cohesive forces which exert the greatest effectwhen residual amounts of liquid remain. The water .or other liquid used.

7 remaining in the core, represents the irreducible ,liqu-id present vinthe formation.

Weight readings are taken at suitably spaced intervals of time, andthese transferred to volumetric readings, are plotted on cartesianco-ordinates as a function of time which results in a curve such as isshown in Fig. 3.

The data for the curves, Figs. '3 and 4, were obtained. by saturating'test'plug No. 8 (shown in Table I) with tetrachloroethan'e, wa-terfandtoluene; the points A, Band C of Fig. 3 indicate the inflection pointcorresponding to'the irreducible liquid content for the various fluidsrespectively. The points A, B, and C referred to :are each the measureof the maximum amount of fluid remaining as an adsorbed 'film upon theinternal surfaces of the sample after [free evaporation of the bulkliquid from the interstitial spaces has occurred. Such'remaining surfacefilm of liquid is not free-flowing :nor is it subject to freeevaporation but rather to a much slower desorptive vaporization. Thisdesorption region of the curve is substantially linear with time.Therefore, to graphically determine' the point of irreducible liquidcontent requires that a sufiicient number of readings be taken .in thedesorption region to plot a straight line. This straight line such asa-a, bb', or 0-0, in Fig. 3 is then extended to the left as shown. Thetangent or inflection points A, B, and C thus Y :maximum :quantity-ofunflowable =.adsorbediiuuid will remain substantially constant andreproduciwble within the normal variations :of .room "item-- formed showgraphically where free liquid has disappeared and where desorption ofthe remaining internal surface-adsorbed liquid begins. The inflectionpoint values are tabulated below together with the saturant absorptionfactors and the pore volume of the particular sample under study in Fig.3.

The d column expressing the desired irreducible water value isderived byapplying the data for a given saturant from the first three columnsaccording to the formula,

Reasonably constant temperature conditions "such as ordinarily prevailunder room conditions are satisfactory for the accuracy'of the method.

The reason for this is that the desorptive vaporization of the adsorbedliquid film is relatively slow and little affected by temperature changein comparison'with'theinitialevaporation of tree 'Iliquid.Thustheinfiectionpoint marking the perature.

The core may .be considered as being made up of two prinicipal parts:.(:1) the sand grains :held together by a cementing material and :(2)the pore volume or free space between the :grains. The pore volume .maybe considered :as :being :composed of capillaries capable oftransmitting soil, gas, .or water, and :a certain volume occupied by theirreducible water retained .by "the smaller capillaries. This latterwater, which is to "be determined, will .not flow "from the core sampleeven when flushed with an oil, natural gas, or other reservoir fluid.

It is to be noted that the treated core sample It may be placed eitherin a glass tube II as illustrated, or simply in front of a funnelthrough which air is passed. The saturated sample l0 mayrbe suspendedinair at room temperature and pressure with no air blowing around it.

Another apparatus which may be used in the practice of my improvedmethod is that illustrated in Fig. 2 in which the numeral 53 representsa container used as an enclosure for the apparatus. Tube 1.1, providedwith Valve 19 admits air, or other suitable gas to the container 3. Thecontainer 53 is provided with valved exhaust tubes is which may beconnected to a vacuum pump or merely left open to the atmosphere. Apressure gauge 2?: is provided to indicate the pres sure within thecontainer H5. The core sample 2| is treated in the same manner with aliquid suchas wateror other vaporizable liquid as before described, andis held suspended from a coiled fiat spring it by means of holder orclamp 14. The coiled flat spring it; is rigidly attached to the top ofcontainer i3 and has a small mirror [6 fastened thereto .50 that anymovement of the spring .15 is directly transmitted to the mirror I6. Alight'beam is directed on the mirror 66. As the sample changes weight,the mirror is moved by consequent change in position of the spring andby passing .thebeam of light reflected by the mirror 16 through a windowin .theiside of the container l3 and. onto a photographic film that ismoving'w'ith constant velocity at right angles to the rotation .of themirror, the evaporation rate of the liquid from the sample may berecorded. Such photographic equipment hasnot been'illustrated as .it isfelt that it is well known to those skilled in the art and furthermention is not necessary. The determination of the irreducible water isthen accomplished in the same manner 'asbefore'd'escribed.

till

At this point it may be noted that when the liquid used has a vaporpressure less than atm'ospheric'at room temperature, the valved outlettubes i8 may be'left open'to the atmosphere tior connected toa vacuumpump if the evaporation rate is to "be accelerated by having thepressure within the container at less than atmospheric. if the fluidused hasa'vapor pressure greaterithan atmospheric at roomtemperature,.thenthe valves '13 and 19 may be so adjusted that thepressure withinthe container is greater than atmospheric. Regardless ofthe 'specificliquid used, the control of the pressure within thecontainer |.3 may be utilized to control the evaporation rate of theliquid .from the sample. By usinga liquid ,havfine a vapor pressuregreater than atmospheric at room temperatures .or by suitablapressurecontrol over the pressure in .the containertheliguid may be caused toevaporate atthe desired .r.a'te

without the necessity of blowing a gas onto the sample.

Table I given below lists the results of tests made with various liquidsand types of core samples. The agreement in the values of irreduciblewater shows that any of the liquids mentioned may be used in thisevaporation method.

Furthermore, the evaporation method is equally applicable to coresamples over a wide range of permeability to air as will be noted inTable 1. Samples having very low air permeability, e. g., Nos. 7 and 8of Table I, are subject to erratic results when tested by other methods.

up of fluid passing through the small capillaries and spreading over thesurface of crystals too greatly separated to be classified asmicrocapillaries. The energy causing this flow has been referred to asimmersional wetting and expresses the free energy change when a solidsurface in equilibrium with the vapors of the liquid is covered by theliquid.

To calculate the capillary pressure, consider this energy as causing aflow of liquid to be hydrostatically balanced by an applied capillarypressure at any saturation. The following equation can be derived fromthe calculation of the capillary pressure and a typical curve, as ob-Table I IRREDUCIBLE WATER OF CORES Weight Air Perme- Irreduciblo l PorePorosity of ability Water (per- Plug No. I mmation Sample Vtglglle(351;. (mum saturating Liquid cent pore (grams) darcys) volume)letrachlorethane 6. 5 1 Dolomite 10.573 2. 13 39. 4 5, 123 Toluene 6. 7Benzene 7. 4 Tetraclilorethane.. S. 2 2 Sandy Dolomite. 10. 451 1.36 25.7 2, 648 8. 3 8. 2 10.8 3 Sandstone 12. 829 1. 13 1o. 0 539 Toluene 11.6Benzene 10.9 Tetraehlorethane. 7. 4 4 .do 12. 054 1. O0 19. 1 295Toluene 7. 9 N-Propyl Alcohol. 7. 5 when... 17.0 5 do 14. 416 1. 37 20.6 115 Tetraehlorcthane.... 17. 7 Toluene l7. 3 'letrachlorethanan. l6. 86 Limestone 20. 657 1.53 17. 9 l9 Toluene l7. 1 Benzene. 17. 0 7 do 13461 1 1 1s 2 3m 27. 2 8 -d0 13. 380 1.03 16. 9 28. 4 27. 8

Table II IRREDUCIBLE WATER USING NORMAL SIZE PLUGS AND CORE CHIPS AirPerm- Irreducible Weight of Pore Core eabihty Water (per- SampleFormation vrgicnine (ml-1m saturating Liquid cent Pore darcys) volume)19. 407 2. 79 674 Tetrachlorcthane 16. 9 9 1. 966 .29 674 d 16.7 22. 6222. 75 825 Toluene 14. 0 10 do 2.415 .33 825 -do 13.6 2. 415 S3 825Tetraehlorethane. 13. 7 13.360 1. 75 16 .d0 16.8 11 do .r 2.252 .33 16.do 17.4 2. 252 33 16 Toluene l7. 2 12 do 21. 853 2. 03 10Tetraehlorethane.. 23. 6 2.908 .27 10 do 24.0

As before mentioned, my method of determining irreducible water can bemade on either standard plugs with flat faces, or on pieces which may besmaller with irregular surfaces which are generally known as core chips.Table II given above shows the agreement of the evaporation method,testing the same sample either as a normal test plug or as a chip ofapproximately 2 grams.

By my method of determining irreducible water, capillary pressures, andother related characteristics of the core sample, these properties arederived from the observed data, calculated by a hydrodynamic treatmentof the data. From a standpoint of surface relationship, the test sampleundergoing a saturation change by evaporation is also experiencing theflow of fluid from the inner section to the exposed surface. This flowis made tained by this method, is shown in Figure 4, which was derivedfor core sample 8 shown in Table I.

aSAA, n Tr.

Where The change in internal surface area of the rock can be calculatedfrom the liquid flow deter mined from the curves represented by Figure3. This method is, therefore, also a means of obtaining data for thecalculation of the internal 9 surface area of the rock exposed to fluidsavailable to flow.

An observed data curve such as shown in Figure 3 gives the liquid volumecontent as a function of time and may be represented by a decay equationof the form:

V =Vo where V=volume of liquid in core sample at any time (t) Vo=volumeof liquid in core sample when i=0,

i. e., at start of evaporation test a=an exponential constant of theequation depending on the characteristics of the core sample.

From a hydrostatic consideration it can be mathematically shown that theconstant (a) of the equation is the resistance of the core sample tofluid flow and therefore an inverse function of its permeability. Byapplying the correct units to the constants associated with the exponent(a), the permeability can be calculated in millidarcys. My method,therefore, enables the obtaining of data sufficient for the calculationsof the permeability of the core sample.

Since the core surface is exposed in all directions, the permeabilityfound by this method is more representative of the ability of the rockstructure to produce oil and/or gas than the permeability determined bymethods that are unidirectional determinations.

From the foregoing description it can be seen that the present inventioncomprises steps of obtaining a sample, saturating it with a vaporizableliquid, and evaporating the liquid from the core sample while takingaccurate weight determinations at suitably spaced intervals of time. Forthe purposes of this invention it has been found that an overallevaporation time of approximately one hour affords the desired results.However, it is to be noted that this time limit is not critical and maybe of any duration that is practical, while still affording a rapiddetermination. It is also possible to determine, with the data obtainedby this method, (1) the irreducible water of a porous rock sample, (2)the permeability of a porous medium, and (3) internal rock surface areaand (4) the capillary pressure. This invention may be valued by the factthat the data required for calculations is obtained within one hourwhile these values by methods heretofore used require several days.

Other modes of applying the principle of the invention may be employed,change being made as regards the details described, provided thefeatures stated in any of the following claims, or the equivalent ofsuch, be employed.

I therefore particularly point out and distinct- 1y claim as myinvention:

1. A method for determining the irreducible water of a subsurfacestratum comprising the steps of saturating a core sample taken from saidstratum with a vaporizable liquid of known properties, evaporating theliquid from said sample, periodically weighing the sample during theevaporation, plotting a rate of evaporation curve from the findings,determining the inflection point on the curve, and determining theirreducible water of said sample therefrom.

2. A method for the determination of the irreducible water of a sampletaken from the subsurface strata comprising the steps of treatin thesample to remove approximately all of its petroleum content, saturatingsaid sample with a volatile liquid of known properties, evaporating theliquid from said sample, and determining the irreducible water of saidsample from the inflection point on a plotted rate of evaporation curve.

3. A method for the determination of the irreducible water of a sampletaken from the subsurface strata comprising the steps of treating thesample to remove approximately all of its petroleum content, saturatingsaid sample with tetrachloroethane, evaporating said tetrachlorethanefrom said sample, and determining the irreducible water of said samplefrom the inflection point on a plotted rate of evaporation curve.

4. A method for the determination of the irreducible water of a sampletaken from the subsurface strata comprising the steps of treating thesample to remove approximately all of its petroleum content, saturatingsaid sample with toluene, evaporating said toluene from said sample, anddetermining the irreducible water of said sample from the inflectionpoint on a plotted rate of evaporation curve.

5. A method for the determination of the irreducible water of a sampletaken from the subsurface strata comprising the steps of treating thesample to remove approximately all of its petroleum content, saturatingsaid sample with benzene, evaporating said benzene from said sample, anddetermining the irreducible water of said sample from the inflectionpoint on a plotted rate of evaporation curve.

6. A method for the determination of the irreducible water of a sampletaken from the subsurface strata comprising the steps of treating thesample'to remove approximately all of its petroleum content, saturatingsaid sample with n-propyl alcohol, evaporating said n-propyl alcoholfrom said sample, and determining the irreducible water of said samplefrom the inflection point on a plotted rate of evaporation curve.

7. A method for the determination of the irreducible water of a sampletaken from the subsurface strata comprising the steps of treating thesample to remove approximately all of its petroleum content, saturatingsaid sample with water, evaporating said added water from said sample,and determining the irreducible water of said sample from the inflectionpoint on a. plotted rate of evaporation curve.

8. A method for the determination of the irreducible water and otherphysical properties of a sample taken from the sub-surface stratacomprising the steps of treating the sample to remove approximately allof its petroleum content, saturating said sample with an inert, residuefree volatile liquid of known properties, evaporating the liquid by theexposure of said saturated sample to a stream of inert vapor,determining from the inflection of the rate of the plotted evaporationcurve the point at which irreducible water remains, and calculating fromthe curve other physical properties of the subsurface strata representedby said sample thereof.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 648,868 Hartshome May 1, 1900 694,782 Prinz Mar. 4, 19022,359,278 Allen et a1 Oct. 3, 1944

